Toward a direct combination of space-geodetic techniques at the measurement level: Methodology and main issues

In the framework of the activities of the Combination Research Centers ( CRC) of the International Earth Rotation and Reference Systems Service ( IERS), the French Groupe de Recherche en Geodesie Spatiale ( GRGS) studies the benefit of combining space-geodetic techniques ( Doppler orbitography and radiopositioning integrated by satellite, GPS, satellite laser ranging, and very long baseline interferometry) at the observational level. This combination aims to produce a global and consistent solution for Earth orientation parameters ( EOPs), polar motion x(p) and y(p), and universal time UT1 with a 1-day or a 6-hour sampling, as well as weekly station positions. In this paper we present a methodology for multitechnique combination at the observational level. We process the measurements of the four techniques over a 1-year period ( the year 2002) in order to illustrate and validate our method. All techniques are processed with the same computational framework, thus with the same models and a priori values for parameters. By using the same software and conventions, we avoid inconsistencies in individual computations. We process each technique individually and inside the combination. The comparison between these solutions is a way of analyzing the power of our method even if the actual status of our software does not reproduce the state-of-the-art analyses of each technique. However, we produce an analysis of the quality of our individual computations so that readers can get an informed appreciation of the current capabilities of our software. Finally, we present the capability of such combinations in terms of accuracy and precision, we underline the main issues of our method and propose solutions to solve them in the future.